1. Field of the Invention
The present invention relates to a cellulose ester film, an optically compensatory film, a polarizing plate, and a liquid crystal display device.
2. Description of Related Art
In liquid crystal display devices, the use of a transparent polymer film as an optically compensatory film for the purposes of enlarging a viewing angle, improving image coloration, and enhancing contrast is a widely known technology. In a most diffused VA (vertically aligned) mode, TN mode, or the like, optically compensatory films capable of controlling particularly optical characteristics (for example, an Re value and an Rth value) to desired values is being demanded.
On the other hand, a cellulose ester film is widely employed in optically compensatory films, the polarizing plates, and the image display devices since films which are more excellent from the standpoints of planarity and uniformity and which have a suitable water vapor permeability can be prepared from the cellulose ester film. Further, a cellulose ester film can be adhered directly on-line with the most general a polarizer including polyvinyl alcohol (PVA)/iodine. For that reason, the cellulose ester, in particular, cellulose acetate, is widely employed as a protective film of a polarizing plate.
On the other hand, a solution film-forming process is widely utilized as a method for preparing a cellulose ester film to be used for optical applications. In this case, for the purpose of imparting high-speed film-forming adaptability during the preparation, it is preferred to add a plasticizer. This is because by adding a plasticizer, it is possible to volatilize the solvent within a short period of time during drying at the time of solution film-forming.
However, in a transparent polymer film containing a plasticizer which is usually used, there is, for example, the case where, when the polymer film is treated at a high temperature in a drying step or the like, smoking is generated; failures in operations are caused due to the attachment of a volatilized oil or the like to a manufacturing machine; or area defects are generated due to the attachment of stains to the polymer film. For this reason, there were naturally restrictions on the preparation condition or treatment condition relative to the transparent polymer film using a plasticizer.
Moreover, in the case where a transparent polymer film such as a cellulose ester film and the like is used for optical applications such as an optically compensatory film, a support of an optically compensatory film, a protective film of a polarizing plate, and a liquid crystal display device, it is a very important factor to control the optical anisotropy in determining the performance of a display device (for example, visibility). For that reason, development of a film which has high development of optical characteristics, hardly suffers from contamination at the time of preparation, and hardly generates failure of the surface state is being demanded.
WO 05/061595 discloses a stretched cellulose ester film which contains at least one kind of an ester-based compound having an aromatic terminal consisting of an alkylene dicarboxylic acid residue having 4 to 12 carbon atoms, an alkylene glycol having 2 to 12 carbon atoms, and a benzene monocarboxylic acid. In addition, it is described that the use of an aromatic terminal ester-based compound has little breakage during the difficult production, has little adherence failure or dent failure upon long-term storage or transportation, and improves dimensional stability, and planarity.
JP-A-61-276836 discloses a cellulose derivative resin composition which contains a polyester consisting of a phthalic acid-based polyester and a dihydric alcohol, describing the improvement of irreversibility, non-volatility, and non-migrating property.
JP-A-2006-64803 discloses a stretched cellulose film which is a cellulose ester film containing a polyester polyol obtained from a glycol having an average number of carbon atoms of 2 to 3.5 and a (anhydrous) dihydrochloric acid having an average number of carbon atoms of 4 to 5.5, wherein the value of in-plane retardation is from 30 to 200 nm and the value of retardation in the thickness direction is in the range from 70 to 400 nm, describing the improvement of humidity stability.
JP-A-2007-3767 discloses a technology in which a polyester and a polyester ether, each having a mass average molecular weight of 400 to 5000, are added for the purpose of the preparation of a cellulose ester film which is excellent in prevention of raw material deposition, water vapor permeability, and dimension degree.
In addition, Plastic Zairyo Koza (Lecture on Plastic Material) (Vol. 17, Nikkan Kogyo Shinbun Sha, “Seniso-kei Jushi (Fiber-Based Resin)”, p. 121, year of 1970) discloses triester phosphates such as triphenyl phosphate, phthalate esters, or the like as a plasticizer of a cellulose acylate film.